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APSC 278 (Engineering Materials) > Polymers I > Flashcards

Polymers I Flashcards

1
Q

What is a polymer?

A

long chain of molecules made up of a series of repeating units

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2
Q

What is mer

A

repeating unit

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3
Q

What is the backbone of the polymer chain?

A

A series of atoms joined by
strong covalent bonds, most commonly carbon-carbon

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4
Q

What does the random rotation of covalent bonds in the backbone of a polymer chain lead to?

A

Bends,
kinks and coils in the polymer chain

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5
Q

What role do the backbone and addition of side groups play in differentiating polymers?

A

The backbone and addition of side groups distinguish the one polymer
from another

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6
Q

What are plastics?

A

Synthetic or semi-synthetic
polymers

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7
Q

What have plastics been historically generated from?

A

Oil or natural gas

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8
Q

What are common sources that plastics are now generated from?

A

Oil or natural gas
wood fibers
corn
organic materials (banana peels!)

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9
Q

What are natural polymers?

A

Polymers
derived from existing biological
processes

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10
Q

What are examples of natural polymers?

A

-Cellulose
-Sugars and starches (polysaccharides)
-DNA
-Rubber

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11
Q

What is cis-polyisoprene derived from?

A

Sap of some plants and trees

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12
Q

What is unique about cellulose

A

It’s an organic material used for cell walls of plants and trees.

It’s the most abundant organic material on earth

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13
Q

Which chemical is natural rubber?

A

cis-polyisoprene

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14
Q

How are polymers differentiated?

A

By their side groups and/or backbone
chain

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15
Q

What type of impact can small differences in polymers have?

A

Small differences can greatly
impact properties (melting points,
reactivity, brittleness, stiffness)

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16
Q

What are many properties of polymers linked to?

A

the polymer chain length
and molecular weight

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17
Q

What happens to the length of polymer chains during polymerization?

A

Polymer chains grow to different lengths –there is a distribution of chain lengths

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18
Q

What are the 2 ways of defining the molecular weight?

A

Number-average molecular weight
Weight-average molecular weight

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19
Q

How is the number-average molecular weight determined?

A

Determined from a histogram of
the number of chains within a given size interval

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20
Q

How is the weight-average molecular weight determined?

A

Determined from a histogram of
the total mass of the chains within the given size interval

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21
Q

What is true in all cases about the number- vs. weight-average molecular weight?

A

In all cases, the weight-average molecular
weight will be higher than the number-average.

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22
Q

What is the histogram for the weight fraction more heavily weighted towards?

A

The histogram for the weight fraction is more heavily
weighted to the longer, heavier chains.

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23
Q

What is another way of expressing average chain size of a polymer?

A

degree of polymerisation(DP)

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24
Q

What does the degree of polymerization represent?

A

The average # of mer units in a chain

25
Q

How is the degree of polymerization calculated?

A

DP is typically calculated from the number-average molecular weight.

26
Q

How does the degree of polymerization impact melting points, stiffness, and strength?

A

Higher DP means:
* Higher melting points
* Higher stiffness
* Higher strength

27
Q

What are linear polymers?

A

Polymers with mer units joined end to end in single chains

28
Q

What are branched polymers?

A

Polymers with side-branched chains connected to the main ones

29
Q

How do branched polymers result?

A

From side reactions that occur during
synthesis of polymer

30
Q

How do branched polymers affect the chain packing efficiency and polymer density?

A

Chain packing efficiency is reduced and lowers polymer density

31
Q

Can polymers with linear structures be branched?

A

Yes, polymers that form linear structures may also
be branched.

32
Q

Describe the branching of high-density polyethylene (HDPE)

A

Low degree of short-chain branching; primarily a linear polymer.

33
Q

What is the density of HDPE?

A

The density of greater or
equal to 0.941 g/cm^3

34
Q

Describe the properties of HDPE

A

-Low degree of short-chain branching

The mostly linear molecules pack together
well, so intermolecular forces are stronger
than in highly branched polymers.

35
Q

Describe the commercial applications of HDPE.

A

Used in products and packaging such as
milk jugs, detergent bottles, butter tubs,
garbage containers, and water pipes.

36
Q

What proportion of toys are manufactured from HDPE?

A

One-third of all toys are manufactured
from HDPE

37
Q

Describe the branching of low-density polyethylene (LDPE)

A

High degree of short-chain branching + long-chain branching

38
Q

What is the density range of LDPE?

A

0.910–0.940 g/cm3

39
Q

Describe the main properties of LDPE?

A

LDPE has a high degree of short- and long-chain branching, which means that chains
pack poorly.

It has, therefore, less strong
intermolecular forces, resulting in a lower
tensile strength and increased ductility.

40
Q

What are the commercial applications of LDPE?

A

Used for both rigid containers and plastic
film applications such as plastic bags and
film wrap

41
Q

What happens during crosslinking?

A

adjacent linear chains strongly
joined to one another

42
Q

When is crosslinking achieved?

A

During synthesis of a polymer or by a nonreversible chemical reaction

43
Q

How is crosslinking often accomplished?

A

By additive atoms or
molecules that are covalently bonded to
the chains

44
Q

What is a common property among rubber elastic materials?

A

Many rubber elastic materials are
irreversibly crosslinked (called
vulcanization)

45
Q

What is alginate?

A

A naturally occurring
polymer, typically derived from brown
algae

46
Q

What are the advantages of alginate?

A

Low cost, biocompatibility
and mild gelation conditions

47
Q

How is hydrogel formed upon cross-linking?

A

The block copolymer of guluronate and
mannuronate residues to create hydrogel. The guluronate cross links with Ca2+ to form an egg box structure.

48
Q

What is the result of guluronate cross-linking with divalent cations (Ex. Ca2+)

A

An egg-box structure

49
Q

Why does molecular weight or degree of polymerization affect the mechanical properties of polymers? (Written Response)

A

*Greater van der Waals interactions between polymer chains
*Greater thermal energy and stress required to disrupt these bonds
*Giving rise to higher melting point and higher strength (lower ductility)

50
Q

What are some defining characteristics of polymers?

A

*Lightweight (density = 0.9 - 3 g/cm^3)
*Low elastic modulus (in bulk form)
*Low fracture toughness (in bulk form)
*Good ductility and specific toughness (in bulk form)
*Highly formable (in bulk form)
*Chemistry is well understood and manipulatable

51
Q

Which of these substances are polymers?: Hemaglobin, polylactic acid (PLA), DNA, collagen, and silicone.

A

All of them are polymers.

52
Q

What is the relationship between the size of a side chain group, bond rotation, and tensile strength?

A

Larger side groups limit ease of bond rotation (i.e. make it harder), thereby increasing strength

53
Q

What properties of polymers can decrease ductility and increase strength?

A

-Degree of crosslinking
-Number-average molecular weight
-Degree of polymerization

54
Q

Why does the polymer strength decrease due to the degree of polymerization decreasing?

A

Due to reduced secondary bonding between polymer chains.

55
Q

What is the relationship between the average number of monomer units in a chain and the melting point of the chain?

A

As the average number of monomer units in a chain increases, the melting point of the chain increases.

56
Q

What happens to the ductility when you increase the degree of crosslinking in rubber elastic materials?

A

The ductility decreases.

57
Q

What is the relationship between temperature, young’s modulus, ultimate tensile strength, and ductility?

A

As temperature increases, the young’s modulus and ultimate tensile strength decreases and ductility increases.

58
Q

How many times can polymers stretch beyond the original length before breaking?

A

They stretch over ten times the original length before breaking.

59
Q

Is it possible to grind up and reuse phenol formaldehyde and polyethylene polymers?

A

It’s impossible to grind up and reuse phenol formaldehyde since it is a thermoset and not amenable to remolding.

It’s possible to grind up and reuse polyethylene since it is a thermoplastic and can be remolded

APSC 278 (Engineering Materials) (30 decks)

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